I am so sorry to read this. Personally, I have only heard of 3 reactions to Rabies vaccines. One got very ill but recovered. Two died but from Anaphylactic shock. Unfortunately, any animal can react to any vaccine just like humans. I don't have time right now, but you might do a search and read up on Rabies vaccination reactions. I am so sorry.
I have never known a dog to die that long after receiving a rabies vaccine and have the death be caused by the vaccine.
Rabies vaccines are killed vaccines, meaning the virus that is being injected is not viable and therefore not able to cause disease. An anaphylactic reaction usually happens almost immediately after being injected, and if not almost immediately, certainly within 24 hours.
Unfortunately, the only way to tell for sure what the cause of death was would be a necropsy. Short of that only your vet's best educated guess based on the symptoms he observed will be possible.
Your other dog is showing normal signs of grieving for his friend. It has been proven that animals do understand death in their own way, and it is becoming more and more of a practice (when possible) to allow the other animals in the household to see and smell the body of the deceased pet before cremating or burying it. This helps them to understand why the deceased pet is no longer there and they are better able to get on with their lives. Until your surviving pet begins to act more like his old self pay some special attention to him. Go for more walks, brush him frequently and give a few more treats than normal until you see his old personality return.
I am very sorry for your loss.
My baby died last week after getting her rabies shots. She was 18 months old and weighed 5lbs. The Vet said there was no way the shots killed her but she was totally healthy until she got her shots. I noticed a change immediately, I took her back to the vet in 4 days, she died in my arms the 7 day. My baby was for my depression and worked better than any meds on the market. She was a Fawn Chihuahua,
may be a reaction due to vaccination, your dog was allergic to this preparation